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DEVELOPMENT OF HIGH TEMPRTATURE DESORPTION SYSTEM IN THE ADSORPTIVE CONCENTRATOR

FOR HEAVY COMPONENTS OF VOC (VOLATILE ORGANIC COMPOUNDS)

Ken­ichiro Yamada, Keimei Furuki, Yuuji Fujioka, Hiroshi Okano and Tsutomu Hirose Seibu Giken Co., Ltd., Fukuoka, Japan

Introduction

Volatile organic compounds (VOC) emitted in a large volume from chemical plants, semiconductor industries, printing and coating processes and so on has been recognized as one of precursors for suspended particle matters and photochemical oxidants. The governmental regulations for the VOC emission are getting more and more severe in many countries in a worldwide movement to the environmental protection. For example in Japan, the air pollution control law revised in 2004 have been enforced in April 2006 and the aim is to reduce the annual emission in 2010 is aimed to be reduced by 30% relative to the total emission of 1.5million tons in 2000 by a combination of the governmental regulations and voluntary efforts of emission sites.

High efficiency units for VOC abatement are required to meet the above target of the air pollution control. VOC of low concentration are destructed finally by direct or catalytic combustion, corona destruction, or ozone decomposition unless the recovery by cooling or compression is economical in the case of high concentration (>1%). VOC­laden air is strongly recommended to pretreated to concentrate VOC and reduce the load of the final destruction units. Preconcentrators are usually based on adsorption on various porous solids, although absorption and membrane processes are not impossible. Remarkable progress in adsorptive VOC treatments in the recent one or two decades may be the introduction of thermal swing honeycomb rotor adsorbers into the market. This type of VOC concentrators were characterized by monolith or honeycomb structure of adsorbent instead of the conventional particles or pellets and by a rapid response to temperature swing.

Seibu Giken Co., Ltd, Fukuoka, Japan, put the VOC concentrators into the market in 1988 as an extension of their corrugation technology developed in honeycomb rotor dehumidifiers and regenerative heat exchangers. The present authors have discussed the efficiency of the thermal swing VOC concentrator in a conference paper by Kuma et al. 1) and in journals by Mitsuma et al. 2) and Yamauchi et al. 3) They described the details of honeycomb rotor adsorbers developed by Seibu Giken Co. Ltd and derived some general principles for design and operation of honeycomb rotor VOC concentrators. The most recent development is discussed in another paper 4) of this AIChE Annual Meeting.

One of current issues is the deterioration of efficiency due to a long term running. Exhaust gas involves sometimes VOC of high boiling point or VOC easy to polymerize. When these VOCs are processed in the concentrator, they are difficult to be desorbed at the standard temperature and accumulated in the rotor to cause the deterioration of the removal efficiency during a long term operation for years. We developed recently the reactivation technology for the deteriorated adsorbent rotor by a high temperature desorption together with heat proof sealing. The purpose of the present paper is to discuss a new reactivation system to extend the life time of the honeycomb rotor adsorbent.

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Honeycomb Rotor Adsorbent and VOC Destruction System

Figure 1 shows the total VOC destruction system combined with the adsorptive concentrator and the thermal oxidizer (combustion) and Fig. 2 shows a 3­D diagram of the concentrator. VOC­laden air is concentrated by a factor of about 10 in a honeycomb rotor concentrator according to the flow sheet given in arrowed lines before it is combusted directly in an oxidizer to destruct it finally. The degree of enrichment is usually sufficient high for combustion of VOC. The direct combustion sometimes is replaced by catalytic combustion or others when VOC concentration in the feed air is too low to concentrate it up to the sufficient concentration. Combustion heat released is utilized through heat exchange to regenerate the honeycomb adsorbent by desorbing the adsorbed VOC at an elevated temperature. Thus a self­sufficient system can be constructed without any external heat source.

A housing holding a slowly rotating honeycomb adsorbent rotor is subdivided usually into three sectors as shown in Fig. 2. The first is a process zone where VOC vapor was removed from the feed air by adsorption onto the adsorbent surface during passing through narrow channels of the honeycomb structure as shown in Fig. 3. The second is a regeneration zone where the adsorbent rotor is regenerated by hot air streamed countercurrently to the process zone. The last is a cooling zone, which is located between the process and regeneration zones and serves to purge the crude product from the bulk of fine product and to cool the rotor to keep a high adsorption capacity in the subsequent process zone. The rotor was kept airtight to avoid air leakage across each zone by a set of radial and circumferential seals, of which the details are discussed by Mitsuma et al. 5) In typical commercial applications, the ratio of cross­sectional area of process: regeneration: cooling zone is set to 10:1:1 and the regeneration temperature is 180.

The key technology of the honeycomb rotor adsorber is the preparation of a high­efficiency adsorbent in a matrix of corrugated sheet; the detailed procedure can be found in patents by Kuma et al. 6­8) A plane strip and a

PROCESS ZONEβ p

COOLING ZONEβ c

REGENERATION ZONEβ r

ENRICHED VOC

CLEAN AIR

FEED AIR

GEARED MOTOR

HEAT EXCHANGER

HEAT EXCHANGER

INCINERATOR

C p0 T p０

U p

FILTER

C r1 T r1 U r

C p1, T p1, U p

HONEYCOMB ROTOR

C r0 T r0 U r

PROCESS ZONEβ p

COOLING ZONEβ c

REGENERATION ZONEβ r

ENRICHED VOC

CLEAN AIR

FEED AIR

GEARED MOTOR

HEAT EXCHANGER

HEAT EXCHANGER

INCINERATOR

C p0 T p０

U p

FILTER

C r1 T r1 U r

C p1, T p1, U p

HONEYCOMB ROTOR

C r0 T r0 U r

Fig. 1 Integrated system of the VOC destruction

200ｐｐｍ

8ｐｐｍ

１920ｐｐｍ

Cooling zone Regeneration heater

Regeneration zone

Regeneration fan

VOC rotor

Rotor drive Process zone

Process fan

Pre­filter 200ｐｐｍ

8ｐｐｍ

１920ｐｐｍ

Cooling zone Regeneration heater

Regeneration zone

Regeneration fan

VOC rotor

Rotor drive Process zone

Process fan

Pre­filter

Fig. 2 A 3­D diagram of the VOC concentrator

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corrugated strip of 0.2mm thick ceramic fiber paper with high voidage were first stacked with heat­resistant adhesive to manufacture a single layer of honeycomb with a typical channel pitch of 3.2mm width x l.7mm height and the wall thickness of 0.2mm. Then the single­layer honeycomb was rolled up with the same adhesive into the shape of a honeycomb rotor as shown in pictures of Fig. 2 up to a desired diameter while the tension applied to the layer was controlled carefully. The largest commercial size is about 4m in diameter, and a large­diameter unit was manufactured by fixing several precut honeycomb segments in a frame instead of the above single stage rolling­up. The rotor was once calcined at about 500 to manufacture a heat­resistant ceramic rotor by removing unnecessary organic matters involved as a binder in the ceramic paper. The ceramic rotor was then impregnated with dilute silica sol in which fine powder of high silica zeolite was suspended. The composite adsorbent of zeolite was solidified in the void of ceramic fiber sheet of the rotor matrix after drying. The adsorbent honeycomb rotor was completed by heating it, cutting it in 450mm width, and finishing the end surface flush. High silica zeolite, i.e. zeolite with high Si/Al ratio, as used here bears a hydrophobic surface to accelerate the adsorbability for organic compounds.

Many commercial units are operating at the present with the removal efficiency higher than 95% and the concentration ratio of 5­20 under the standard specifications of the rotor diameter of 0.3­3m, the rotor width of 0.4­0.45m, air velocity of 2­4m/s, temperature of regeneration air of about 180 C and co­exiting humidity less than 80%. Scientific aspects of the effect of the operating conditions such as the rotation speed, air velocity etc. on the removal efficiency were discussed by Yamauchi et al. 3) .

Deterioration and Reactivation of the Honeycomb Rotor Adsorbent

Exhaust gas from factories related to semiconductors and liquid crystals involves sometimes VOC of low saturated vapor pressure such as DMSO (Dimethyl sulfoxide), MEA (Methyl ethyl amine) etc. or VOC easy to polymerize. When these VOCs are processed in the concentrator incorporated with a hydrophobic zeolite rotor, they are difficult to be desorbed at the standard temperature of 180­200 C and accumulated in the rotor to cause the deterioration of the removal efficiency. In the worst case of miss operation, the accumulated VOC as fuel may burn the rotor down. We arrange the following two measures for this issue; 1) Reactivation by spray washing with water and 2) High temperature reactivation..

Reactivation by Spray Washing with Water. In the case of water soluble VOC such as DMSO and MEA,

the accumulated VOC can be washed out successfully by spraying water on the honeycomb wall as shown

3.2mm

1.7mm

0.2mm 3.2mm

1.7mm

0.2mm 3.2mm

1.7mm

0.2mm

Fig. 3 Cross­sectional view of the honeycomb structure

Fig. 4 Water works in the water spray washing

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in Fig.4 and the performance has been supported by many experiences in the semiconductor factory.

Indications of Adsorption or Adhesion of High Boiling Point Matters A test piece for the visual observation was sliced from a honeycomb rotor of a VOC concentrator

which had operated in a semiconductor factory for 3.5 years. The color of the surface turned to

yellow­brown in the used piece from near white in a virgin sample as compared in Fig. 5. It cannot be decolorized by washing with water, implying the adhesion of some substance which is water­insoluble and hardly volatile at the standard regeneration temperature of 180 C. A microscopic observation shows that voids between zeolite crystals were clogged with something and the contour of particles is blurred as shown in Fig. 6. The differential thermal analysis was carried out for the used sample. An endothermic peak and a corresponding weight loss appeared around 300 C due to the desorption or evaporation of probable high boiling point matters, as shown in Fig.7. These preliminary observation imply that the deteriorated adsorbent rotor may be reactivated by exposure of it in a high temperature atmosphere above 300 C.

High Temperature Reactivation System

Static Adsorption Test The amount adsorbed was compared between a virgin rotor, a deteriorated rotor and a reactivated

rotor to confirm the advantage of a high temperature reactivation. The sample was sliced from each rotor in a 10 mm thick disk at a position of 30 mm depth from the inlet end of the feed. The reactivated rotor is the one which was pre treated at 300 C for 1 hour. Saturated vapor of isopropyl alcohol was adsorbed at 25 C in a desiccator. As shown in Fig. 8, the amount adsorbed of the deteriorated rotor had decreased to 61% of the virgin rotor, but it was recovered to 89% of the virgin by the reactivation at 300 C. With the high temperature reactivation at 300 C, the clogging substance disappeared and the particle contour

[% ] [μV/mg]

0

0 200 400 600

Temperature [ﾟC ]

85

90

95

TGA

­5.0

0.0

5.0

DTA

305 ﾟC

14.8kJ/g

­9.338%

Weight loss

Heating rate:15 /m in

100

Fig. 7 Thermal analysis of a deteriorated sample of the honeycomb rotor

Fig. 6 Microscopic observation of the surface of the zeolite honeycomb rotor

Air Flow

Deteriorated sample

Virgin sample

(a) Deteriorated in 3.5 years (b) Reactivation at 300C

Fig. 5 Color change on the surface

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became clear as shown in Fig. 6(b). In the thermal analysis of the reactivated sample, the endothermic peak in Fig. 7 disappeared and the weight loss decreased to 2.8% from 9.3% for the deteriorated one.

Onsite reactivation system In the case of water insoluble VOC

accumulated, spray washing with water is not effective but the reactivation by hot air stream is desirable. Off­site operation, of course, is possible to reactivate by high temperature heating in a separate factory as done with active carbon concentrators. However, it is not a practical process in view of the dismantling expenditure and intermission of operation. The present reactivation at high temperature enabled to reactivate the rotor on site of the VOC concentrator in relatively short time.

A flow sheet of the onsite reactivation system is shown in Fig. 9. A blue line refers to the usual operation (On­line) as a VOC concentrator while a red line refers to the case of Off­line for the high temperature reactivation. In the actual operation of reactivation, the feed air is stopped and the outdoor air is heated to 300 C by a burner after heat exchange to reactivate the deteriorated rotor which rotates very slowly at a few round per hour.

Heat Proof Seal The heat­resistant seal was essential in developing the reactivation system to stand high

temperature reactivation. The rotor surface is rubbed usually with flexible seal to prevent the concentrated air from leakage to/from other zones but the heat resistant rubber seal used in the standard specification could not stand a high temperature of 300 C necessary for reactivation. On the other hand, inorganic sealing material is highly heat resistant but has some troubles with easy wear of the rotor surface and less tightness of sealing. We developed in 2005 a hybrid seal mechanism by combining metal seal, Teflon­like seal and heat proof rubber with high seal performance to provide a heat resistant, wear proof, and tight proof seal successfully. Figure 10 shows the seal structure at a boundary between the regeneration zone and the cooling zone. A pair of heat protector plate of Teflon was installed at the tip of a clearance adjuster so that the clearance between the plate tip and the rotor end is kept at a minimum value. Layers of air between the protector plates each other and between the plate and a rubber seal work as thermal insulator to prevent the rubber seal from thermal damage.

0

1

2

3

4

5

6

Used Rotor Used Rotor Virgin Rotor

Sample Rotor

Amou

nt Adsorbedq*x10

0 [kg/kg]

　　 ( Not Reactivated) ( Reactivated at 300) ( Reactivated at 300 )

Fig. 8 Static adsorption test to confirm the effect of high temperature reactivation

Desorp.

Cooling

Process Zone

TIC

Heat Exchanger

300C

Process Fan

VOC Rotor

Out Air

open

close

From THO Desorption Fan

On­Line Off­Line

open

Burner Desorp.

Cooling

Process Zone

TIC

Heat Exchanger

300C

Process Fan

VOC Rotor

Out Air

open

close

From THO Desorption Fan

On­Line Off­Line

open

Burner Desorp.

Cooling

Process Zone

TIC

Heat Exchanger

300C

Process Fan

VOC Rotor

Out Air

open

close

From THO Desorption Fan

On­Line Off­Line

open

Burner

Fig. 9 Flow sheet of the onsite reactivation system by high temperature air stream

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Temperature was measured at various points on the rotor surface, i.e. points ①­⑤ in Fig.10, to examine the seal performance. Temperature increases with increasing rotation speed as shown in Fig. 11(a). A rotation speed at 2 rph is slow enough to keep low temperature since the rubber seal is exposed below 200 C as shown in Fig. 11(b).

Two Staged Reactivation System. The single operation of either water

spray washing or high temperature reactivation cannot be expected to be sufficient for reactivation when both water soluble and water insoluble VOC are

accumulated in the honeycomb rotor. A two­staged reactivation system was developed for such a case by carrying out first the water spray washing followed by the high temperature reactivation. This system enabled to treat the exhaust gas having been impossible to be treated so far due to various kinds of hardly desorbed VOCs involved.

Concluding Remarks

The performance deterioration due to hardly desorbed compounds and the reactivation by high temperature air stream were discussed and some new findings were obtained. 1) An differential thermal analysis showed that the hardly desorbed compounds can be desorbed at

about 300 C. The amount adsorbed of was recovered to about 90% of the virgin rotor when it was treated at 300 C.

Air flow

(b) A­A’ cross­section

Rotor surface

REGENERATION ZONE

Heat protector

COOLING ZONE

Supporting plate

Rubber seal

Seal holder

Clearance adjustment

Air flow

Clearance adjustment

Holder

Cooling zone

Regeneration zone

A’ A

Heat protector

Rubber seal

(a) Top view of the rotor

⑤

④ ③ ① ② Air flow

(b) A­A’ cross­section

Rotor surface

REGENERATION ZONE

Heat protector

COOLING ZONE

Supporting plate

Rubber seal

Seal holder

Clearance adjustment

Air flow

Clearance adjustment

Holder

Cooling zone

Regeneration zone

A’ A

Heat protector

Rubber seal

(a) Top view of the rotor

Air flow

(b) A­A’ cross­section

Rotor surface

REGENERATION ZONE

Heat protector

COOLING ZONE

Supporting plate

Rubber seal

Seal holder

Clearance adjustment

Air flow

Clearance adjustment

Holder

Cooling zone

Regeneration zone

A’ A

Heat protector

Rubber seal

Regeneration zone

A’ A A’ A

Heat protector

Rubber seal

(a) Top view of the rotor

⑤

④ ③ ① ②

Fig. 10 A new mechanism of heat proof seal between the regeneration and cooling zones

(a) Effect of the rotation speed (b) Temperature profile

Fig. 11 Performance of the new heat proof seal

180

200

220

240

260

0 5 10 15

Rotation speed [rph]

Temperature [

]

50

100

150

200

250

300

350

0 2 4 6

Position in Fig.10(b)

Temperature [

]

7

2) The onsite reactivation system was developed by passing the hot air of 300 C through the regeneration zone while the process air was stopped. 3) Heat proof seal was developed by the installation of heat protector plates in the regeneration zone to

enable the high temperature reactivation. We are forced to destruct efficiently a large amount of dilute VOC although a powerful technology

has been unavailable and thus much dilute VOC was discharged into the atmosphere so far. The VOC concentrator discussed here is a useful tool to solve the problem and a combination with the thermal oxidizer can lead to the energy saving VOC destruction. Much progress is expected in the environmental measures with the VOC concentrator in developing countries as well as developed ones,

References

(1) Kuma, T., Y. Mitsuma, Y. Ota, T. Hirose, “Removal Efficiency of Volatile Organic Compounds, VOCs, by Ceramic Honeycomb Rotor Adsorbents,” Fundamentals of Adsorption (Procedings of the 5 th International Conference on Fundamentals of Adsorption), 5, 481(1996).

(2) Mitsuma, Y., Y. Ota, T. Hirose, “Performance of Thermal Swing Honeycomb VOC Concentrators,” J. Chem. Eng. Japan, 31, 482(1998)

(3) Yamauchi, H., A.Kodama, T.Hirose, H.Okano, K.Yamada, “Performance of VOC Abatement by Thermal Swing Honeycomb Rotor Adsorbers,” Ind. Eng. Chem. Res., 46, 4316(2007)

(4) Hirose, T., H. Okano, K. Yamada, K. Furuki, Y. Fujioka, “Continuous VOC (Volatile Organic Compounds) Concentrators by Thermal Swing Honeycomb Rotor Adsorbent,” Extended Abstracts of 2007 AIChE Annual Meeting, #470a (Environmental Applications of Adsorption (09013))

(5) Mitsuma, Y., T. Kuma, H. Yamauchi, T. Hirose, “On Improvement and Scaling­up of Thermal Swing Honeycomb Adsorbent for VOC Concentrators”, Kagakukogaku Rombunshu, 24, 248(1998)

(6) Kuma, T., H. Okano, “Active Gas Absorbing Element and Method of Manufacturing”, United States Patent 4886769, 1989.

(7) Kuma, T., “Method of Manufacturing A Gas Absorbing Element or Catalyst Carrier Having A Honeycomb Structure” , United States Patent 5194414, 1993.

(8) Kuma, T., “Gas Adsorbing Element and Method for Forming Same”, United States Patent 5348922, 1994